1. Technical Field
The present disclosure relates to a system for measuring an electroencephalogram of a user who wears on his or her head a housing having electrodes provided thereon. Specifically, the present disclosure relates to an electroencephalogram measurement system for determining states of attachment of electrodes, and estimating an insufficiency cause in accordance with the position of any insufficient electrode.
2. Description of the Related Art
Conventionally, as a method of measuring an electroencephalogram (Electroencephalogram: EEG), a method which records changes in the potentials of two points on the scalp (potentials of a reference electrode and an electrode for measurement) is known. An electroencephalogram is an electrical signal which is measured as a potential difference between the reference electrode and the electrode for measurement. An electroencephalogram is supposed to reflect encephalic activities (electrical activities of the cranial nerve cells), and more specifically, electrical activities of the cerebral cortex. As compared to other methods of measuring encephalic activities in noninvasive manners, the aforementioned method of measuring potential changes as an electroencephalogram has the advantages of high time resolution and easy measurement. At medical institutions, electroencephalogram measurement methods based on electrical potentials have conventionally been used for diagnosis of epilepsy, Alzheimer's disease, and the like. At research institutions, electroencephalogram measurement methods have been utilized as tools for basis research on the process of perception and cognizance, which is information processing in the human brain.
In order to measure an electroencephalogram, it is necessary to attach electrodes on the head. Therefore, a method of electrode attachment will be described. First, paste is applied on an electrode. The paste is a cream having a high electrical conductivity, serving to enhance the electrical conductivity between the skin and the electrode. A specialist (a third person) other than the user ensures that the electrode having the paste applied thereon is in tight contact with the scalp of the user. Conventionally, since the electrode and an electroencephalograph would be connected via a lead wire which permits high freedom, the third person would be able to attach the electrode at any arbitrary position on the head of the user. Moreover, in the case of taking multi-point measurements by utilizing a plurality of electrodes, there exists a method which employs an electrode cap having high contraction and expansion properties. Both methods allow the electrodes to be stably attached at predetermined positions on the head, without being affected by the head shape of the user. For the electrode attachment, about several minutes is required for each electrode, which is a relatively long time. However, the time required for electrode attachment has never been regarded as a problem because, in the past, this has all been in a preparatory procedure for diagnosis or basis research.
In recent years, the downsizing of electroencephalographs and increased accuracy of signal processing techniques are making it possible to develop electroencephalogram interfaces for inferring the psychological state of a user based on an electroencephalogram and inferring his or her intent of manipulation as to how they want to manipulate a device, or intent of selection as to which one of a plurality of options they want to select.
For example, as an electroencephalogram interface for use with a healthy user, Emanuel Donchin and two others, “The Mental Prosthesis: Assessing the Speed of a P300-Based Brain-Computer Interface”, TRANSACTIONS ON REHABILITATION ENGINEERING 2000, Vol. 8, June 2000 discloses an electroencephalogram interface which infers an intent of selection of a user. Among event-related potentials of the electroencephalogram, the electroencephalogram interface of Donchin et al. uses a characteristic signal component called “P300” to distinguish an option that a user wishes to select. As used herein, an “event-related potential” refers to a transient potential fluctuation in the brain, which is a portion of the electroencephalogram and which occurs in temporal relationship with an external or internal event. Moreover, “P300” refers to an event-related potential component, in an electroencephalogram signal at 20 milliseconds to 400 milliseconds as reckoned from a given event, having a positive (plus direction) amplitude peak.
However, in such manners of use, which require lead wires or a special cap to be attached with the help of a specialist, it is difficult for a user in a daily-life environment to wear the electrodes by himself or herself, have their electroencephalogram measured, and have the resultant electroencephalogram used for an electroencephalogram interface or the like. Now, an example will be discussed where an electroencephalogram measurement apparatus (electrodes and an amplifier) is incorporated in a wearable device such as a head-mount display (Head-Mount-Display: HMD) or a headset as a housing for electroencephalogram measurement to measure an electroencephalogram. A housing such as an HMD or a headset, for example, has a high rigidity as compared to the aforementioned lead wires or electrode cap, and is easy to wear. Therefore, it is not difficult for the user himself or herself to, as soon as wearing the HMD or headset, position any electrode for electroencephalogram measurement that is incorporated in the housing so as to be in the neighborhood where measurement is expected to be taken.
In the aforementioned conventional technique, the user himself or herself needs to apply paste on the electrodes. Furthermore, after the device is detached, the paste remaining on the electrodes and in the places where the electrodes have been disposed must be wiped off by the user himself or herself. Therefore, in order for the user to easily wear an electroencephalogram measurement device by himself or herself, it is an example to adopt electrodes which do not require use of paste (hereinafter referred to as “dry electrodes”).
However, use of dry electrodes presents a problem in wearing stability. As one example, when a force acts on a dry electrode, the state of contact between the skin and the dry electrode will change because there is no paste. This results in the electrode position changing even though contact with the skin may be maintained (“electrode shifting”), or the electrode becoming lifted off the skin to create a space between the skin and the electrode, thus disabling electroencephalogram measurement (“electrode disengagement”).
Note that a paste has a high viscosity, and serves not only to enhance the electrical conductivity between the skin and the electrode, but also to prevent electrode shifting and electrode disengagement. This conserves the state of contact between the skin and the electrode even when the position of an electrode is slightly changed due to a force acting thereupon, because the paste with a high viscosity will then be deformed to allow the electroencephalogram to be properly measured.
When electrode shifting occurs, the skin will be rubbing against the electrode surface, so that noises may likely be mixed in the measured electroencephalogram. If an electrode disengagement occurs, electroencephalogram measurement will be so affected that it is made impossible. Since the user will not always be in a resting state but will undergo various motions in a daily-life environment, insufficiencies concerning electrode contact, such as electrode shifting and electrode disengagement, are likely to occur. In the present specification, an electrode which has become insufficient in terms of contact or attachment will be referred to as an “insufficient electrode”. An insufficient electrode is an electrode in a poor state of attachment, i.e., an electrode which is not in a good state of attachment with the skin, such that an electroencephalogram cannot be properly measured.
In order to reduce insufficiencies of measurement due to such electrode shifting and electrode disengagement, it is necessary to quickly detect if a situation obstructing electroencephalogram measurement has occurred. Conventionally known methods for detecting an unfavorable situation for electroencephalogram measurement are: the methods in Japanese Examined Utility Model Publication No. 7-3347 (“hereinafter Patent Document 1”); Japanese Laid-Open Patent Publication No. 2006-212348 (“hereinafter Patent Document 2”); Japanese Laid-Open Patent Publication No. 2006-6665 (“hereinafter Patent Document 3”); and the specification of Japanese Patent No. 4465414 (“hereinafter Patent Document 4”), which will be described below.
Patent Document 1 discloses a method of flowing a weak current through an electroencephalogram electrode, calculating a resistance value (contact resistance) between the skin and the electrode from the measured voltage value, and estimating a state of contact between the skin and the electrode. As a result, insufficiencies concerning the state of electrode attachment are detected (see Patent Document 1, page 3, left column, second paragraph).
In Patent Document 2, a coil is provided near an electrode (FIG. 2 of Patent Document 2), and a voltage is applied to the coil. Based on whether a resultant induced current in the electrode is superposed on the electroencephalogram waveform or not, it is determined as to whether the electrode and the scalp are in contact (Patent paragraph [0038] of Document 2).
In Patent Document 3, measurements are taken of a plurality of “electroencephalogram channels”, each electroencephalogram channel defining an electroencephalogram signal to be measured based on the potential difference between a pair of electrodes. In other words, a plurality of pairs of electrodes are provided, and an electroencephalogram signal is measured by each pair. Then, for each electroencephalogram channel, a Signal (=signal to be measured) and Noise (=any signal other than the signal to be measured) are calculated. Through a comparison of the S/N ratio against a threshold value, it is determined as to which electroencephalogram channel is suffering from a measurement insufficiency (paragraph [0028] of Patent Document 3).
In Patent Document 4, insufficiencies of wearing between electrodes for electroencephalogram measurement is distinguished via impedance checks. This makes it possible to identify whether electrodes are in contact with the skin of the user or not. The impedance check, as used in Patent Document 4, is an approach of flowing a very minute amount of current between two electrodes to measure a value of resistance existing between the places where the two electrodes are in contact with the skin. When electrode disengagement, user perspiration, or the like occurs to prevent proper detection of an electroencephalogram, there is an increase in the resistance value between the electrodes. Therefore, by performing impedance checks to measure resistance values between electrodes, it becomes possible to determine which combination of electrodes fails to attain proper contact (paragraphs [0167], [0168] of Patent Document 4).